1. Field of the Invention
The present invention relates to the use of stable chromium(III) compounds, particularly in the form of colorant solutions, which are used for colouring ceramics.
2. Description of the Related Art
In the ceramics industry, unglazed ceramics have hitherto been coloured and decorated using, inter alia, compounds of the transition metals, e.g. of chromium, cobalt, iron, manganese, nickel , gold, etc., in the form of their water-soluble salts (chlorides, acetates, nitrates or sulphates), as described, for example, in DE-A 21 17 449.
These compounds were selected because they are stable to hydrolysis and oxidation in aqueous solution.
Aqueous chromium compounds having the oxidation state VI, e.g. potassium dichromates, were, in particular, preferably used because they are nitrate- and sulphate-free and thus form no corrosive gases during the firing of the ceramics and, in addition, have a high metal ion concentration in the aqueous solution and are also stable and compatible in mixtures with other metal compounds.
However, apart from the chemical stability, it is just as important that when these compounds are used as ceramic colorant solutions they have the properties desired there. A very important property which these colorant solutions have to have is an ability to penetrate sufficiently deeply into the unglazed ceramic surface. Only in this way is it ensured that the colour produced can develop or form to a sufficient intensity after firing. The penetration depth also has to be sufficiently great, in particular, because the surface of the ceramic piece is subjected to further mechanical treatment, in particular grinding, after firing. Here, a few tenths of a millimeter are taken off the coloured surface material. The ceramic article thus loses its intense colour if the penetration depth was not sufficiently great.
Naturally, the colour intensity also depends on the concentration of the colorant solutions applied. For the present purposes, concentration means the concentration of colour-imparting metal cations. It is clear that the higher this concentration, the more intense the colour which the compound can produce. However, for physicochemical reasons, it is not possible to increase the concentration at will, because otherwise the stability of the solution is no longer ensured. A metal salt solution is referred to as stable if it displays no precipitation or flocculation even at a relatively high ion concentration. An important stability factor is the pH of the solution. It is known that such compounds are always stable only in a particular pH range.
Further requirements which these compounds have to meet are determined by occupational health and safety aspects. The use of these compounds should be as safe as possible without complicated protective measures having to be taken during their use.
However, those skilled in the art know that the use of some of these metal salt solutions is problematical in terms of human health. In these cases, lawmakers have stipulated either particular occupational health and safety measures or the complete replacement of these compounds by others which produce equivalent results. In the case of chromium(VI) compounds, lawmakers have decided that these compounds have to be replaced as from 1998, since the chromium(VI) compounds customarily used are known to be very toxic when swallowed or inhaled; they irritate the mucous membranes and particularly the eyes if they are used in the form of an aerosol and finally they are carcinogenic according to the German Federal Health Department classification. All these properties naturally hinder continued use, or it is ruled out automatically.
It is therefore an object of the invention to find suitable chromium compounds which have the same properties as the chromium(VI) compounds for the required application but do not have the adverse properties indicated above.
This object could surprisingly be achieved by the use of neutralized or partially neutralized chromium(III) coordination compound (complex compound) which are a constituent of ceramic colorant solutions and are obtainable by reacting a chromium(III) compound with an organic dicarboxylic acid of the general formula (COOH)xe2x80x94(CH2)nxe2x80x94(COOH), where the index n is from 0 to 10, or an unsaturated, organic dicarboxylic acid, in particular maleic acid and fumaric acid, by treating the chromium(III) compound with the aqueous solution of the carboxylic acid at temperatures between [sic] 40 to 60xc2x0 C., preferably at 55xc2x0 C., while stirring and, after filtration, adjusting the pH of the solution to from 5.5 to 6.5 using alkali, preferably using concentrated ammonia, where the chromium(III) coordination (complex) compound solution has a chromium content of from 5 to 10% by weight, preferably a chromium content of from 5 to 7% by weight.